Gas turbine engines for aircraft are normally guaranteed for thrust performance. A newly manufactured engine must be tested to determine compliance with the guarantee, for example, a production test "green" run. Thrust measurement is also important during development. Furthermore, later in life, thrust performance testing of an engine is required to evaluate its current status, particularly after a major engine overhaul.
An indoor test cell offers several advantages in performing a test. Such test is less affected by variable wind or unreliable weather conditions and the noise can be attenuated to avoid excessive disturbances.
Certain corrections must be applied to the various test readings in a test cell in order to determine the equivalent outdoor operation. Preferably these corrections should be as low, stable and repetitive as possible to avoid introduction of error. One of the corrections involved is that of the inlet momentum correction.
It is known that under certain conditions inlet vortices can form which are then ingested into the gas turbine engine. This can result in an engine surge event and prevent useful testing from being conducted. In some cases this can even result in compressor blade damage.
Within a bounding stream line the air is accelerated into the entrance of the gas turbine. The secondary airflow between this streamline and the cell walls decelerates with static pressure increase. At a deceleration velocity ratio on the order of 0.4 or 0.5 separation at the cell wall occurs. The separation is non-uniform. This provides sufficient shear gradient in the direction transverse to the flow to promote the formation of a vortex. This vortex may then be ingested into the engine.
Two methods are known to avoid the damaging effects of vortex formation. One approach involves the streamlining of the duct upstream of the engine with a fairing approaching the shape of the outer streamline perimeter directing all of the air flow to the engine inlet. This inherently accelerates the air flow to the full inlet velocity before it enters the engine, thereby requiring a substantial performance inlet momentum correction. Little or no bypass air is required with this approach.
Such a structure is expensive to build, and forces on the structure must be isolated from the measured force. Specific adaption must be made for each engine being tested. Furthermore, this structure can not be easily moved or removed for access to the engine or for changing the engines.
A more customary solution is the provision of adequate bypass flow or secondary flow around the engine. The exhaust from the gas turbine engine enters an exhaust collector passage. This operates as an inductor to draw air into the passage in addition to the exhaust from the gas turbine engine. Accordingly, excess flow is supplied beyond that needed by the gas turbine engine.
Because of this excess flow the velocity ratio is higher and there is less deceleration of the secondary air and a reduced potential for separation at the walls. Furthermore, any separation that does occur is convected downstream of the gas turbine inlet and is not ingested therein.
Experience has shown that bypass ratios greater than 0.75 or 0.8 are acceptable. In order to test an engine of a given thrust and having a particular air flow, at least 1.8 times the engine required flow must be handled by the test cell. For a given test cell size it follows that larger engines have two problems. One, the extremely high air velocities create considerable noise and excessive forces on the various structure required for the test cell. Second, the approach velocities increase requiring substantial thrust correction.
In accordance with prior art concepts the only solution to such a problem is to build a new and larger test cell.
It is an object of the invention to establish a structure which permits the testing of larger higher thrust engines in a given size engine test cell.